EP-A-0 136 871 discloses a pair of small diameter optical fibers (12, 14) arranged in a side-by-side configuration, the first fiber (12) providing a passive guide for pump light, and the second fiber (14) formed of a material, such as Nd:YAG which has amplifying properties at the signal frequency. The signal to be amplified propagates through the second fiber (14) resulting in amplification of the signal. The first fiber (12) is pumped from one or both ends, utilizing a cone shaped rod (50) which is optically coupled thereto. A plurality of pump light sources (60) are mounted on the end face (58) of the rod (50) to introduce collimated pump light into the large end (52) of the cone shaped rod (50).
It is known to improve the coupling efficiency by locating a cylindrical lens between the laser chip and one end of the pigtail, with the cylindrical lens focusing the transverse component of the laser beam into the fiber core. In this respect reference can be made to EP-A-0 565 843 and also to "Hybrid laser-to-fiber coupler with a cylindrical lens" by Maeda M. et al., Applied Optics, Vol. 16, No. 7, July 1977, pp 1966-1970.
Such cylindrical lens does not modify the divergence of the beam in the plane of the slit axis, so that at a proper distance the beam from the laser chip is substantially reduced to having a circular cross section and can easily be coupled to the core of the optical fiber.
The pigtail is further fused to the final fiber, i.e. in general the fiber to be fed or in particular the fiber carrying the signal to be amplified, either at the other end of the feeding pigtail (Y coupler) or along a portion of the feeding pigtail length (X coupler). The coupling ratio of such coupler is proportional to the ratio of the squared diameter of the receiving fiber core to the squared diameter of the receiving fiber core plus the squared diameter of the feeding fiber core.
By using the SM-DC fiber technique an efficient feeding of a single-mode fiber core through a multi-mode fiber pigtail can be achieved. When the multi-mode core of the SM-DC fiber has the same diameter of the feeding fiber (i.e. 60 .mu.m), the coupling ratio is about 0.5. To further increase the coupling ratio to the SM-DC fiber it is necessary to reduce the feeding fiber core diameter. With a diameter of less than 20 microns the coupling ratio could theoretically reach values greater than 0.9.
However, with a reduced core diameter of the multi-mode feeding fiber to improve the efficiency of the light transfer at the coupler, the efficiency of the light transfer from the laser to the feeding fiber would become very poor even using a cylindrical lens.